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Diplodocoid neural spine bifurcation influenced by ontogenetic growth

From: Ben Creisler

A new online paper not yet mentioned on the DML:

D. Cary Woodruff & Denver W. Fowler (2012)
Ontogenetic influence on neural spine bifurcation in diplodocoidea
(dinosauria: Sauropoda): A critical phylogenetic character.
Journal of Morphology (advance online publication)
DOI: 10.1002/jmor.20021

Within Diplodocoidea (Dinosauria: Sauropoda), phylogenetic position of
the three subclades Rebbachisauridae, Dicraeosauridae, and
Diplodocidae is strongly influenced by a relatively small number of
characters. Neural spine bifurcation, especially within the cervical
vertebrae, is considered to be a derived character, with taxa that
lack this feature regarded as relatively basal. Our analysis of dorsal
and cervical vertebrae from small-sized diplodocoids (representing at
least 18 individuals) reveals that neural spine bifurcation is less
well developed or absent in smaller specimens. New preparation of the
roughly 200-cm long diplodocid juvenile Sauriermuseum Aathal 0009
reveals simple nonbifurcated cervical neural spines, strongly
reminiscent of more basal sauropods such as Omeisaurus. An identical
pattern of ontogenetically linked bifurcation has also been observed
in several specimens of the basal macronarian Camarasaurus, suggesting
that this is characteristic of several clades of Sauropoda. We suggest
that neural spine bifurcation performs a biomechanical function
related to horizontal positioning of the neck that may become
significant only at the onset of a larger body size, hence, its
apparent absence or weaker development in smaller specimens. These
results have significant implications for the taxonomy and
phylogenetic position of taxa described from specimens of small body
size. On the basis of shallow bifurcation of its cervical and dorsal
neural spines, the small diplodocid Suuwassea is more parsimoniously
interpreted as an immature specimen of an already recognized
diplodocid taxon. Our findings emphasize the view that nonmature
dinosaurs often exhibit morphologies more similar to their ancestral
state and may therefore occupy a more basal position in phylogenetic
analyses than would mature specimens of the same species. In light of
this, we stress the need for phylogenetic reanalysis of sauropod
clades where vital characters may be ontogenetically variable,
particularly when data is derived from small individuals.